[ELF] - Adjust rangeToString to report ranges in a different format.

It was raised during the review of D43819.

LLD usually use [X, Y] for reporting ranges, like below:
"relocation R_386_16 out of range: 65536 is not in [0, 65535]"

Patch changes rangeToString() to do the same.

Differential revision: https://reviews.llvm.org/D44207

llvm-svn: 326918